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Designing my first tube preamp
Designing my first tube preamp
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Old 6th December 2019, 08:53 AM   #31
petertub is offline petertub  Sweden
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Designing my first tube preamp
Quote:
Originally Posted by Verbstank View Post
Yeah I edited my last post to say I was dumb and used the max current ratings rather than operating condition currents.


Attached an updated schematic if you are still hanging in there Values are substitutes until I work out the math on what they should be.



I had read through that Valve Wizard page previously but it makes much more sense now that you have explained a few things! So his section on Choosing the Load is exactly what I would use to find the value for the load resistor?



And because the cathode follower provides no gain, is that why you were questioning the tone circuit? Because I would not be gaining it back up before the output? If that is the case, would it be better to put the tone circuit in between the two 12ax7s and allow the 2nd 12ax7 stage to go directly to the cathode follower?
As for the cathode follower,

make the cathode follower like the example in walve wizard :
The Valve Wizard -Cathode Follower


470 ohm cathode <> grid, 18k to ground. ECC82 will do just fine!
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Old 6th December 2019, 07:18 PM   #32
Verbstank is offline Verbstank
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Quote:
Originally Posted by petertub View Post
As for the cathode follower,

make the cathode follower like the example in walve wizard :
The Valve Wizard -Cathode Follower


470 ohm cathode <> grid, 18k to ground. ECC82 will do just fine!

Hey Petertub,


Thanks for the confirmation on that. Attached is what I HOPE is a final schematic for this thing. If you could look it over that would be awesome, I think my cathode follower is set correctly, I am just wondering now about the load resistor and bias resistor for the other half of the ECC82 (which I'm going to use to gain back up after the tone stack).



Originally I had this project as 3 tubes powering 2 channels, but now I've just simplified it to one channel, two tubes (12ax7 and 12au7). For the 12au7, is it the same procedure to find the load resistor (ask how much current I want to see at the plate (~8mA), how much voltage I want to see (~200 volts) and so I must drop 100 volts from the B+ of 300, therefore 100v/8mA=12.5k load resistor?) How about the cathode bias resistor for the 12au7 gain stage?
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Old 6th December 2019, 08:29 PM   #33
petertub is offline petertub  Sweden
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Designing my first tube preamp
Verbstank
You could decrease the input cap to the cathode follower, it's very high impedance. The output cap could be increased for drive capability. Load resistor, i still claim 18k is best.
I would however decrease the outermost 1Meg resistor to 100k, this to discharge the
output cap fast at power on / off

The grid resistor 250k is ok, the loading on previous stage is even lower then it seems
as the grid resistor is bootstraped from the cathode.

Finally, a "lead-out" resistor of 100 ohm between the cathode and the output cap is
recommended to avoid instability.

If possible. the ecc82 filaments should be elevated to some 25Volts below cathode ( i would advice the cathode to be at 100Volt ) 200 volts between cathode and filament is way
too much.
This later requrement suggests that a second 6.3V ( or 12V ) winding is needed

to allow the other tubes have their filament around 0V
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Last edited by petertub; 6th December 2019 at 08:33 PM. Reason: Spelling, second filament winding
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Old 6th December 2019, 10:49 PM   #34
Verbstank is offline Verbstank
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Quote:
Originally Posted by petertub View Post
Verbstank
You could decrease the input cap to the cathode follower, it's very high impedance. The output cap could be increased for drive capability. Load resistor, i still claim 18k is best.
I would however decrease the outermost 1Meg resistor to 100k, this to discharge the
output cap fast at power on / off

The grid resistor 250k is ok, the loading on previous stage is even lower then it seems
as the grid resistor is bootstraped from the cathode.

Finally, a "lead-out" resistor of 100 ohm between the cathode and the output cap is
recommended to avoid instability.

If possible. the ecc82 filaments should be elevated to some 25Volts below cathode ( i would advice the cathode to be at 100Volt ) 200 volts between cathode and filament is way
too much.
This later requrement suggests that a second 6.3V ( or 12V ) winding is needed

to allow the other tubes have their filament around 0V

Ammended for those values and attached.


Well so do people not normally run 12ax7s and 12au7s heater on the same winding?


Or is it that the B+ needs to change before it is sent to the 12au7 i.e. do I just put another filter cap + resistor in between the power supply and 12au7 plates and run the tube at a lower voltage? Does this make sense?



I guess I could look for a new transformer but this was a pretty good option given I didn't find many with low HT current for preamp and high 6.3v current for multiple tubes.
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Old 7th December 2019, 04:30 AM   #35
PRR is offline PRR  United States
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Quote:
Originally Posted by Verbstank View Post
..why are we measuring the current across the resistor?...
"Across" is wrong tinking. "Through".

Quote:
Originally Posted by Verbstank View Post
So what happens when you want to measure current at a given point?...
You have to *break* the circuit and insert a current meter.

This is normally difficult. If there is a resistor in the circuit, you can measure the voltage across the resistor and compute the current.

Yes, I think you should step away from the electronics books and get a basic electricity book.
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Old 7th December 2019, 07:42 AM   #36
petertub is offline petertub  Sweden
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Designing my first tube preamp
The reason i recommended a separate filament circuit for the 12au7 is that the cathode will
have > 100 Volt . There is a limit how many volts are allowed between filament and cathode
(the insulation is minute and has limits). Thus the 12au7 filament should have a DC offset
to minimize the voltage between cathode and filament. This offset might be obtained be a

large resistor from the cathode to the filament in question.
If you don't have another winding, one option would be to replace the 12au7 with a FET
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Old 8th December 2019, 10:54 PM   #37
kevinkr is offline kevinkr  United States
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Moved to Instruments & Amps since this is a bass pre-amp.
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Old 10th December 2019, 07:52 PM   #38
Mark Tillotson is offline Mark Tillotson
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Running all the stages from the same rail could be an issue - normally you'd string RC decoupling along the chain both to prevent oscillatary feedback paths through the supply rail, and to allow the input tubes to run at lower voltage. The latter reason isn't relevant here I think, but letting the output tube pump current directly into the same supply node as feeds the input tube is worrying - depends how much gain you have.
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Old 11th December 2019, 03:57 AM   #39
Verbstank is offline Verbstank
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Quote:
Originally Posted by Mark Tillotson View Post
Running all the stages from the same rail could be an issue - normally you'd string RC decoupling along the chain both to prevent oscillatary feedback paths through the supply rail, and to allow the input tubes to run at lower voltage. The latter reason isn't relevant here I think, but letting the output tube pump current directly into the same supply node as feeds the input tube is worrying - depends how much gain you have.

Attached is schematic how it stands now,


So if I understand correctly, running the input 12ax7 stage on the same supply as the 12au7 cathode follower is the problem (is the CF being called the "output tube"?) because as the CF operates it will send unwanted feedback back into the supply line and therefore end up in the 12ax7 input stage as well? And having another filter cap + resistor between the CF supply and the rest of the supply is what you're recommending?



Sorry, I am still very new to all this.
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Old 12th December 2019, 01:20 AM   #40
PRR is offline PRR  United States
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